Japan’s Hokkaido Hit by Magnitude 6.2 Quake as Aftershock Advisory Nears End

Seismic activity off Japan’s northeast coast is again drawing attention after a magnitude 6.2 earthquake struck Hokkaido early on the day an “aftershock advisory” issued following last week’s major quake was set to end.

The Japan Meteorological Agency said the quake occurred at about 5:23 a.m. on April 27, centered in southern Tokachi in Hokkaido. It registered a maximum intensity of “upper 5” on Japan’s seismic intensity scale in Urahoro. The intensity scale reflects how strongly shaking is felt in a given area, unlike magnitude, which measures the energy released. An upper-5 reading can disrupt most people’s actions and topple unsecured furniture.

The quake’s depth was about 83 kilometers (52 miles). No major damage had been confirmed as of the agency’s report, and Hokkaido Electric Power Co. said it had found no abnormalities at the Tomari nuclear power plant.

The tremor came exactly one week after a magnitude 7.7 earthquake struck off Sanriku on April 20. After that quake, the Japanese government and the meteorological agency said the likelihood of another strong quake was higher than usual and issued an “aftershock advisory” for 182 municipalities across seven prefectures from Hokkaido to Chiba. The advisory is maintained for about a week, and the Hokkaido quake coincided with its scheduled end.

The meteorological agency said the two quakes did not appear to be directly linked. “This earthquake does not appear to have a direct relationship with the April 20 earthquake,” it said at a news conference, adding that it was “not covered by the aftershock advisory.” The agency also warned that “for about the next week, caution is needed for earthquakes of a similar magnitude.”

Japan introduced the aftershock advisory system after the 2011 Great East Japan Earthquake, when a magnitude 7.3 quake struck two days earlier. The system has been in operation since December 2022. It is issued when a magnitude 7-class or larger quake occurs in designated offshore areas such as the Japan Trench and the Kuril Trench, based on statistics showing the probability of a major quake within seven days rises from about 0.1% to about 1%. It is a probabilistic warning aimed at strengthening preparedness while maintaining daily life.

Nikkei reported the latest advisory was the second, following one in December 2025, and said 182 areas expected to face shaking of at least “lower 6” intensity or a tsunami of 3 meters (about 10 feet) or higher were designated for disaster-response measures.

The Pacific coast from Sanriku to Hokkaido sits on an active boundary where the Pacific Plate subducts beneath the landward plate, allowing stress to build until it is released in earthquakes.

The Asahi Shimbun, citing a briefing by Tohoku University’s International Research Institute of Disaster Science, reported that earthquake swarms began around November last year off Sanriku and that a “slow slip” — gradual movement along the plate boundary — continued until just before the April 20 quake. Slow slip can ease deformation but can also add pressure to nearby faults. Tohoku University professor Fumiaki Tomita said the slow slip may have helped trigger the magnitude 7.7 quake, according to the report.

Asahi also cited an analysis by a team led by Shizuoka Prefectural University specially appointed professor Kazuyoshi Nanjo, saying the “b-value,” a metric used to quantify earthquake patterns, has been particularly low offshore near Hokkaido. A low b-value is seen as a sign of stress accumulation and was reported to overlap with a decades-long “gap” zone where large quakes have not occurred. A similar tendency was observed off Sanriku before the 2011 disaster, the paper said.

Asahi reported that Japan’s Headquarters for Earthquake Research Promotion said in December 2017 that it was “highly urgent” that a magnitude 8.8 or larger megaquake could occur in the region. The assessment was based on analyses of past tsunami deposits indicating such events recur on average every 340 to 380 years, and nearly 400 years have passed since the last one in the early 17th century. Asahi said two papers published in February supported that “urgency” with modern observational data, and that recent abstracts from universities and research institutions have included phrases such as “a state requiring caution continues” and “a recurrence may be imminent.” The newspaper said researchers refer to this centuries-long repetition of megaquakes as a “supercycle.”

Asahi also cited a Tohoku University team’s five-year seafloor observations off Nemuro, saying the Pacific Plate and the upper plate were moving at nearly the same speed and direction and were strongly locked. The team said deformation accumulated since the early 17th-century megaquake may already have reached a level capable of producing a quake of similar size, concluding it should be considered “not strange no matter when it happens.”

The report noted this is different in character from the “Nankai Trough megaquake,” a separate type of potential megaquake expected off Japan’s southeast coast.

Even as warnings persist, public response remains a concern. Asahi said a Cabinet Office survey of 3,500 residents in advisory areas after the first advisory in December found 80% had seen the information, but only 8% said they had prepared to evacuate immediately. More than half took no action despite hearing the advisory, the paper said. Tohoku University professor Shosuke Sato, noting the start of the Golden Week holiday period, said there was no need to change travel plans but urged people to confirm evacuation sites at their destinations in advance.

Misinformation also spread on social media, Asahi reported. It said that after the April 20 quake, videos filmed during the 2024 Noto Peninsula earthquake circulated on X, formerly Twitter, with captions claiming they showed “the moment the quake hit,” and tsunami footage from the 2011 disaster was also shared as if it were current. Posts also circulated an image that appeared to be generated by AI alongside an unfounded claim that “the likelihood of an aftershock is highest around 3 p.m. on the 21st.” Asahi said accounts posting the videos were based not only in Japan but also in South Asia, Europe and China, and many posts appeared to be automatic translations of foreign languages. The Yomiuri Shimbun reported that the Internal Affairs and Communications Ministry asked five major social media operators, including Google, Meta and X, on the day of the quake to take steps to prevent the spread of false information.

Experts caution against treating any single quake as proof that a catastrophe is imminent, noting that earthquakes are inherently difficult to predict precisely in timing and size. Still, they say observational data indicate energy is building underground, and they urge people to begin preparedness now even without forecasting a specific date.

* This article has been translated by AI.

Copyright ⓒ Aju Press All rights reserved.